Abstract: Microbial biopesticides targeting the two-spotted spider mite, Tetranychus urticae, might become an alternative to broad-spectrum chemicals and thus contribute to sustainable crop production. The aim of this paper was to assess the efficacy of entomopathogenic fungus Isaria fumosorosea (syn. Paecilomyces fumosoroseus) spray applications against T. urticae under both laboratory and greenhouse conditions. Blastospores of were obtained by submerged cultivation in growth media using an orbital shaker. In the laboratory experiment, blastospores were applied at concentration 4 × 107/ml using Potter spray tower to leaf discs with spider mite eggs or adult females. In the small scale greenhouse trial the fungus suspension was applied twice to bean plants infested with spider mites using a hand sprayer. The results revealed that under laboratory conditions T. urticae females showed higher susceptibility to the pathogen (92.1% mortality) compared to eggs (53.5% mortality). Symptoms of mycosis were observed on most of cadavers. Greenhouse experiment failed to confirm these results and no effect on spider mite density was found compared to control. The likely reason was extremely high temperature during summer period when the experiment was carried out.